For various reasons, including safety and health concerns, it is necessary to detect metallic contaminants in various non-metallic consumer goods such as food, pharmaceutical or textile products. Typically, coil based metal detectors are used to scan such products for unwanted inclusions of metals or other electrolytes. Signals created by various metals or products as they pass through or by the coils of a metal detector can be split into a resistive and a reactive component, according to the conductivity and magnetic permeability of the metal and product. When the particles are small, the signal from ferrous metal (iron) is primarily reactive, while the signal from stainless steel is primarily resistive.
US2003/0105600 describes an apparatus for detecting contamination of an object by a metal or other electrolyte. The apparatus according to US2003/0105600 comprises a metal detector associated with a computer programmed to compare the observed signal provided by the metal detector with a stored calibration signal corresponding to an object with an acceptable level of contamination, and to identify when an observed signal received from the metal detector deviates by an unacceptable amount from that calibration signal.
U.S. Pat. No. 5,045,789 describes a detector for detecting foreign matter in an object by using discriminating electromagnetic parameters. According to U.S. Pat. No. 5,045,789, in a coordinate system showing the resistive (in phase) and reactive (90° phase delay) components of a detection signal, an elliptic region is defined to represent expected signal values for a product without contaminants. Detection signals with a value or a vector lying outside this elliptic region are judged to represent products which contain foreign matter. Defining an elliptic detection envelope based on actual measurements of the product has the advantage that the elliptic detection envelope is aligned with the product signal, i.e. product compensation is achieved in that the phase of the detection envelope is aligned with the phase of the product. Consequently, even the signal of a highly conductive product, such as meat or cheese, does not pass outside the detection envelope. However, as a negative consequence, large signals from stainless steel are needed to pass outside the detection envelope and, thus, the detector becomes less sensitive to these metals.